The present invention relates to a method and apparatus for radiation detection. It finds particular application in improving the performance of Geiger-Mueller tubes and counters and will be described in conjunction therewith.
Geiger-Mueller tubes have commonly included a gas filled cylindrical metal cathode and a fine wire anode extending axially therethrough. The gas is typically a noble gas, such as argon or neon, which is ionized by a particle passing therethrough causing a uniform discharge between the cathode and anode. The noble gas is mixed with a quench gas which inhibits secondary ionization. Common quench gases include chlorine and bromine which, of course, are highly corrosive to many metals.
Geiger-Mueller tubes find utility in numerous fields. For example, Geiger-Mueller tubes have been used to monitor sources of naturally occuring or artificially introduced radiation. In one application, the radiation from radioactive tracers is introduced into the human body or other biological tissue to monitor the movement or absorption of the tracer therethrough. In another application, the monitored radiation from artifically activated substances is used or introduced into various chemical processing systems, mechanical parts, or the like. Monitoring the radiation from the activated substance indicates the progress of chemical reactions, the movement or migration of the activated substance, the removal, or wear, or erosion of the activated substance, or the like. In yet another application, natural radiation is detected in prospecting for naturally radioactive substances, and the like.
Geiger-Mueller tubes have also been utilized in conjunction with a dedicated radiation source, such as a gamma radiation source, for monitoring a physical substance thereadjacent. For example, accurate measurements are made of the flow rate of oil or fuel flowing through a tube disposed between the radiation source and the Geiger-Mueller tube. As another example, the thickness of sheet goods passing between the source of radiation and Geiger-Mueller tubes is measured. In well logging, the radiation source irradiates surrounding strata and the Geiger-Mueller tube detects reflected and secondary radiation emanating from the surrounding walls of the well.
Although the prior art Geiger-Mueller tubes have been successful in monitoring the above-referenced and other radiation events, their performance tended to degrade at elevated temperatures. One technique for conditioning Geiger-Mueller tubes to operate at higher temperatures was described in U.S. Pat. No. 3,342,538 in which an oxygen plasma was utilized to oxidize the interior surface of the cathode during the manufacturing operation. In another technique described in U.S. Pat. No. 3,892,990, the interior surface of the cathode was coated with a thin layer of chromium, platinum, or a nickel-copper alloy. Of these coatings, the platinum produced the highest sensitivity to gamma radiation.
However, problems were encountered in adhering the platinum to the inner surface of the cathode. Further, the porosity of the platinum tended to permit the corrosive halogen gases to pass therethrough and attack the cathode. At higher temperatures, the halogen degradation of the cathode was accelerated.
U.S. Pat. No. 4,359,661 provided improved resistance to halogens by replacing the platinum coating with layers of chromium oxide and tungsten. Because tungsten is not amenable to plating, a sleeve of tungsten foil having edges and seams was fashioned into a liner for the cathode. The gaseous mixture confined in the cathode chamber tended to migrate between the tungsten foil and the cathode causing partial delamination and degradation of the tube performance. Moreover, the greater thickness of tungsten foil relative to plated platinum, tended to reduce the relative performance characteristics of the tube.
The present invention provides a new and improved radiation detector which overcomes the above-referenced problems and others, yet provides excellent radiation detection characteristics.